1. Field of the Invention
The present invention generally relates to computer connections in a local area network, and more particularly to node devices having Media Dependent Interfaces (MDI) that are installed in such networks and their automatic connection configuration.
2. Background Art
Many local area network products (LAN) use a medium consisting of twisted copper wire pairs for the transmission and reception of data. At a LAN node, one or more twisted pairs transmit data to a second LAN node, and one or more twisted pairs receive data from the second LAN node. This arrangement permits simultaneous data transmission and reception, also called full duplex communications.
In a conventional LAN node with full duplex communications, different twisted pairs are used for transmission and reception. This requires that each end of the link have a defined assignment for its twisted pair connector. For example, a Network Interface Card (NIC) is usually employed as an end node device, while a switch or a repeater will be employed as a concentrator or central component in a star-based network. During link operation, the NIC transmits data on a pair of wires that are connected to the receiver of the hub or switch, and the NIC receives data on a pair of wires that are connected to the transmitter of the hub or switch. If the NIC transmit pair of wires are inadvertently connected to the hub transmit connector, then the communication link will fail. Similarly, if the NIC receive transmit pair of wires are inadvertently connected to the hub receive connector, then the communications link will also fail.
Most LAN standards assign specific connector pins to the wires in the twisted pair to prevent the transmit and receive twisted pairs from being crossed at one of the nodes. In the IEEE-802.3 10BASE-T standard, pins 1 and 2 at an end node are connected to the transmit twisted pair, and pins 3 and 6 are connected to the twisted receive pair. At the hub, which is typically a repeater or a switch for a 10BASE-T network, pins 3 and 6 are connected to the transmit pair, and pins 1 and 2 are connected to the receive pair. This works quite well for configurations where NIC""s are attached to repeaters. However, other configurations are possible with the emergence of switched networks. For example, two NICs can be directly connected together, or two repeaters can be directly connected together, or a repeater can be connected to a switch. Depending upon the pin assignment of these devices, it may become necessary to employ a crossover cable to connect two LAN devices that have incompatible pin assignments.
A manual switch can be employed to switch the connections for the transmit and receive pairs for a hub, repeater, or switch. The switch allows the products to connect to other similar devices but requires an installer to manually push a button. The manual approach works well for limited applications like a repeater-to-repeater links, but not in the general case where it is desired to build a LAN device that attaches to either a repeater, a NIC, or a switch, without manual intervention.
What is needed is an automated means of switching transmitter or receiver connections for a generalized LAN device, including a hub, repeater, or a NIC.
The present invention includes a line driver that is used to selectively drive one of two transmission lines. The line driver is a differential amplifier connected to first and second differential switches. The first differential switch is connected between an output of the differential amplifier and a first of two transmission lines. The second differential switch is connected to the output of the differential amplifier and to the second of two transmission lines. The first and second differential switches are controlled by respective first and second control signals. The output of the differential amplifier is connected to either the first or the second transmission line in response to the first and second control signals.
Power down loopback protection is provided by passive devices that prevent an external network signal from feeding through the unpowered line driver differential switches and injecting a false signal into the network.
An advantage of the present invention is more efficient use of valuable integrated circuit space by using a line driver in place of two line drivers.
Another advantage is the use of a single differential amplifier to drive both output transmission lines. This feature greatly reduces or eliminates the need for post assembly line balancing required when two independent line drivers are used.
A further advantage is the excellent signal isolation achieved by coupling a signal to the input of two parallel switches, then maintaining one switch closed while opening the other.
Another advantage is the mitigation of power down loopback signals without adding additional circuitry or using additional chip space. The protection is passive and does not consume any additional power.
Further features and advantages of the present invention, as well as the structure and operation of various embodiments of the present invention are described in detail below.